VOLUME 11, NUMBER 4
JULY/AUGUST 1997
© Copyright 1997 American Chemical Society
Reviews Petroleum Geochemistry: Concepts, Applications, and Results R. Paul Philp* and Laurence Mansuy School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma Received October 8, 1996. Revised Manuscript Received May 6, 1997X
Petroleum geochemistry has played an important role in many areas of exploration and production for fossil fuels. Many of the more recent developments can be seen to have developed in parallel with developments in analytical chemistry such as gas chromatography and gas chromatography-mass spectrometry. For the past two decades such analytical techniques have been used to search for trace amounts of compounds known as biomarkers present in oils and source rock extracts which can be used to provide valuable information on the origin and history of the oil. In the past two or three years much more effort has been placed on the development and utilization of such techniques as an aid to solving reservoir and production problems. In this paper it is proposed to provide an overview of major developments that have occurred in a number of areas of geochemistry in recent years. This will include developments in reservoir geochemistry such as the use of high-resolution gas chromatography for reservoir continuity studies and high-temperature gas chromatography for characterization of wax deposits. A brief overview of recent developments in biomarker geochemistry will be provided in the section on exploration geochemistry along wth a discussion on the use of various pyrolysis techniques for the purposes of artificial maturation or characterization of the insoluble organic matter in source rocks or asphaltenes in oils. While this paper is not intended for the specialist in geochemistry, it is designed to provide the interested reader with a broad overview of the areas of geochemistry where the significant developments have occurred and continue to occur. As our analytical capabilities increase so do our abilities to obtain a far more detailed and comprehensive picture on the origin of fossil fuels than could ever have been imagined a mere two decades ago.
Introduction The concept of organic geochemistry has been with us for many decades, if not centuries, in one form or another. For example, the presence of asphalt blocks in the Dead Sea many centuries ago was indicative of petroleum accumulations in the area, although the people resident in the area at that time had no idea of the significance of these asphalt blocks, in terms of our current petroleum-based economy. However in the early part of the 20th Century a significant change X
Abstract published in Advance ACS Abstracts, July 1, 1997.
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occurred. Alfreid Triebs, a German chemist, identified the presence of porphyrins in crude oils and realized the structural relationship between these compounds and naturally occurring chlorophyll. This relationship was a very important step in establishing the theory of a biogenic origin for crude oils. From the classical work of Treibs in the 1920s and 1930s it was a quantum leap to the 1970s and developments in analytical work directly associated with the lunar project and Viking missions to Mars. The search for extraterrestrial life was a major driving force behind the development of the hyphenated analytical technique of gas chromatog© 1997 American Chemical Society
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raphy-mass spectrometry which, for the first time, permitted both separation and identification of individual organic compounds in very complex mixtures, exactly the impetus geochemistry required to move forward into the decades which followed. A parallel development was the concept of biomarkers, biological markers, or chemical fossils, as proposed by Calvin and Eglinton in their classic Scientific American paper.1 In brief, it was proposed that there are many compounds in the geological record, generally hydrocarbons, that can be structurally related to their functionalized precursors occurring in living organisms or plants. This relationship, once established, therefore permits one to look for the presence of certain hydrocarbons in the geological record and make inferences concerning the type of source material responsible for the original sediments and possibly the nature of the depositional environment. More recent efforts have also demonstrated that certain compounds can be associated with specific organisms evolving at a certain time during the geological record, and hence the presence of these compounds can also be used to constrain the age of the source rock. The advances that were made in the 1970s came, in a large part, from a great deal of work looking at organic compounds in recent sediments where it was relatively easy to associate specific organic compounds with specific sources of organic matter. However in the mid1970s Wolfgang Seifert, interestingly enough a student of Treibs but then working with Chevron Oil in California, realized the significance and potential importance of biomarkers and their application to petroleumrelated problems. Other oil companies may claim responsibility for applying geochemistry and biomarkers to such problems, but a look at the published record will show beyond any doubt that it was the work of Seifert and his collaborators that really took hold of the concept and demonstrated that it would work in a way that is now being used routinely by oil companies on a world wide basis. Analytical techniques continue to improve in terms of sensitivity and resolution and carbon number range. Much of the early work was done by simply looking at compounds containing up to about 40 carbon atoms. With the advent of high-temperature gas chromatography this range has now been extended to 120 carbon atoms, potentially opening up a whole new arena of geochemical opportunities. New techniques, for example gas chromatography-isotope ratio mass spectrometry, continue to come along and shed new light on the origin and significance of specific biomarkers. This multidisciplinary subject will continue to find new applications and continue to expand. An article such as this, while general in nature, will provide some insight into areas where geochemistry has and will continue to play an important role. Petroleum geochemistry has developed rapidly over the past two or three decades and as illustrated in the monograph by Peters and Moldowan2 and more recently in the outstanding book Petroleum Geochemistry and Geology by Hunt3 the emphasis has been directed (1) Eglinton, G.; Calvin, M. Sci. Am. 1967, 261, 23-32. (2) Peters, K. E.; Moldowan, J. M. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments; Prentice Hall: Englewood Cliffs, NJ, 1992. (3) Hunt, J. M. Petroleum Geochemistry and Geology, 2nd ed.; W. H. Freeman and Company: New York, 1995.
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mainly toward exploration problems. The pace of this development has inevitably slowed and in the past two or three years there has been a marked tendency to apply geochemical concepts to areas concerned with the exploitation and production of fossil fuels. Such areas were previously considered the domain of petroleum and reservoir engineers but the development of reservoir geochemistry has lead to its use in the investigation of many production and reservoir characterization problems. These applications are exceedingly important when it is remembered that in general two-thirds of the oil in a reservoir remains in place, even following secondary and tertiary recovery techniques. Processes responsible for poor production include reservoir heterogeneity; the formation of petroleum-derived barriers, such as tar mats, asphaltenes, bitumens or waxes; interactions between the oil and the mineral matrix; low viscosity of waxy oils; and petroleum biodegradation. An understanding of the processes responsible for the emplacement of petroleum fluids, the nature of the inorganic/organic interactions in the reservoir, and a knowledge of variations in petroleum compositions with time during initial production and secondary and tertiary recovery processes will lead to improved methods for the recovery of residual oils, over and above those currently available. In addition more effective reservoir management will also result from a better understanding of reservoir geochemical problems. Petroleum exploration also requires the support of predictive techniques to provide a clear picture of the mechanisms of oil generation occurring in a given sedimentary basin. The prediction of the quantity and quality of hydrocarbons generated by organic matter as well as the timing of generation are of primary importance to reduce the risks involved in petroleum exploration. The empirical approach to natural maturation can only provide partial information since naturally occurring series describing the whole process of maturation from diagenesis to metagenesis are very rare. Moreover, no complete mass balance can be obtained from these data since migration of hydrocarbons and gases can lead to an underestimation of the amount of oil generated. As a consequence, laboratory pyrolysis studies have become an important tool to simulate maturation processes of organic matter and provide information on the timing, quantity, and nature of hydrocarbons and gases generated as well as on the behavior of the residual kerogen. It is proposed to briefly review some recent developments in reservoir geochemistry and to briefly discuss some developments in petroleum geochemistry, specifically aspects of biomarker geochemistry and the utilization of pyrolysis techniques for simulation of maturation. Reservoir Geochemistry A very important aspect of reservoir management is a knowledge of continuity within a reservoir to assist in determination of the placement of additional production wells. Determination of reservoir continuity, mixing, or comingling of oils from different sources can be readily investigated by whole oil gas chromatography. Oils within a continuous reservoir will have similar chromatographic fingerprints while those from separate reservoirs, or different compartments within a single
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Figure 1. Whole gas chromatograms of five crude oils taken from the same field and same formation but different wells.
reservoir, will have different fingerprints, albeit very subtle, even if the oils are derived from the same source rock. The observed differences, based on the measurement of ratios of several pairs of adjacent peaks, can be treated statistically to differentiate samples derived
from different producing horizons.4 For example the whole oil chromatograms shown in Figure 1 appear to be virtually identical for all of the samples. When peak (4) Hwang, R. J.; Ahmed, A. S.; Moldowan, J. M. Org. Geochem. 1994, 21, 171-188.
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Figure 2. Star plot of ratios from 13 pairs of minor components from each of the five oils shown in Figure 1. From this plot it is possible to propose that these oils can be divided into three groups that are not in contact with each other within the reservoir. The groups are oils A and B; oil C; and oils D and E.
ratios for pairs of several minor components are measured and plotted on a star diagram, oils coming from different formations, not in communication with each other, can be differentiated (Figure 2). Geochemistry also plays an important role in predicting what types of oils may be associated with wax problems and the timing of the onset of wax deposition. Wax deposition generally results from changes to the supercritical character of petroleum fluids during accumulation and production. The temperatures in deeper reservoirs may exceed the critical temperature of the low molecular weight petroleum constituents, e.g. methane and ethane, and these compounds may act as supercritical solvents for the high molecular weight hydrocarbons. Loss of reservoir pressure during production will result in a reduction of the carrying capacity of the supercritical solvent system and will lead to wax precipitation. The waxes which form typically contain a relatively high concentration of high molecular weight hydrocarbons above C30, and possibly up to C100, which may not be observable in the oils collected at the wellhead5-8 (Figure 3). The study of higher molecular weight (C40) hydrocarbons had been largely overlooked prior to the development of high-temperature gas chromatography (HTGC) columns9,10 and supercritical fluid (5) Del Rio, J.-C.; Philp, R. P. Trends Anal. Chem. 1992, 11, 187193. (6) Del Rio, J.-C.; Philp, R. P. Org. Geochem. 1992, 18, 869-880. (7) Del Rio, J.-C.; Philp, R. P.; Allen, J. Org. Geochem. 1992, 18, 541-553. (8) Carlson, R. M.; Moldowan, J. M.; Gallegos, E. J.; Peters, K. E.; Smith, K S.; Seetoo, W. C. In 15th International Meeting on Organic Geochemistry-Oral Communications, Manchester, U.K., September 16-20, 1991. (9) Lipsky, S. R.; Duffy, M. L. J. High Resolut. Chromatogr., Chromatogr. Commun. 1986, 9, 376-382. (10) Lipsky, S. R.; Duffy, M. L. J. High Resolut. Chromatogr., Chromatogr. Commun. 1986, 9, 725-730.
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chromatography11-13 which now permit characterization of hydrocarbons extending to the C120.10,11 The solids may fill in the pores or restrict the pore throats in reservoirs and change reservoir quality predictions in play assessments and basin evaluations. In the laboratory, addition of solvents such as acetone, or the use of cold fingers, can be used to precipitate and quantify the wax content of an oil. The waxes can be analyzed by HTGC and individual compounds quantified by use of appropriate standards. Results from the concentration of a wax from an oil collected from the Anadarko Basin in Oklahoma are shown in Figure 4 and compared with the original oil. Data obtained in this manner can ultimately be used in models developed to accurately predict the cloud point of oils. In addition to the reservoir, waxes as well as asphaltenes, may be deposited in the production tubing or pipelines leading to blockages as far along as the storage tanks. Asphaltenes, typically defined as “material that precipitates out of a crude oil on the addition of excess light n-alkanes”,14 have also been extensively investigated by geochemical techniques. In-reservoir asphaltene deposition leads to the formation of tar mats or sharply defined reservoir zones containing petroleum enriched in asphaltenes relative to the composition of the main oil-leg (see Wilhelms and Larter15,16 for a review on the formation of tar mats and associated properties and problems). The Iatroscan system has been used extensively in the North Sea region as a rapid means of mapping reservoirs laterally and vertically to demonstrate the presence of several relatively thin tar mat regions which would have otherwise been undetected and which in turn could cause major problems associated with production. The application of geochemical techniques to reservoir problems has the potential to significantly improve production and reservoir management. However, as with all of these applications, it will require a great effort for the geochemists to learn the language of the engineers and vice versa if this is to develop and bear significant fruit in terms of improved production. Exploration Geochemistry The term exploration geochemistry covers issues related to generation of oil and its migration from the source rock to the reservoir. Reference to the recent book by Hunt2 will illustrate the diversity of this topic. However, one topic that has been extremely important in exploration geochemistry is the use of biomarkers for source determination, maturity, characterization of depositional environments, determination of the extent of biodegradation, and correlation of oils with their suspected source rocks.17 All of this information forms an important part of any exploration and development program, and when combined with geological and geo(11) Hawthorne, S. B.; Miller, D. J. J. Chromatogr. 1987, 388, 397409. (12) Hawthorne, S. B.; Miller, D. J. J. Chromatogr. 1989, 468, 115125. (13) Carnahan, N. F. J. Pet. Technol. 1989, 41, 1024-1025. (14) Speight, J. G.; Moschopedis, S. E. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; Advances in Chemistry Series 195; American Chemical Society: Washington, DC, 1989, p 1-15. (15) Wilhelms, A.; Larter, S. R. Mar. Pet. Geol. 1994, 11, 418-441. (16) Wilhelms, A.; Larter, S. R. Mar. Pet. Geol. 1994, 11, 442-456. (17) Philp, R. P. Fossil Fuel Biomarkers. Applications and Spectra; Elsevier: Amsterdam, 1985; p 294.
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Figure 3. Waxes from four different wells in Oklahoma analyzed by high-temperature gas chromatography. If these samples had been analyzed by conventional gas chromatography, the components in the carbon number range above C35 would not have been observed, and the seriousness of the problem resulting from the presence of these waxes would have been underestimated.
physical information can produce a comprehensive picture of the basin being evaluated. In the following sections a brief overview of the information obtainable from various classes of biomarkers will be discussed. This is not intended to be a comprehensive review but will give the reader an indication of the type of geochemical information available from these compounds. Biomarkers most frequently used in petroleum geochemistry are generally hydrocarbons derived from functionalized precursors occurring in living systems. Porphyrins and organosulfur compounds are also used in some situations but will not be discussed in this article. n-Alkanes. The n-alkanes are the most extensively studied group of biomarkers since they can be readily analyzed by GC alone. Alkanes are derived from a variety of sources, and their carbon number distributions have been widely used to differentiate marine vs terrestrial source materials. The odd/even carbon number preference of these distributions will change systematically with maturity, and biodegradation will also change the distribution by initially removing the lower number carbon compounds preferentially over the longer chain compounds. An early paper by Hedburg18 described a notable characteristic of oils derived from terrigenous source materials as being their waxy nature resulting from the high wax content of their higher plant source materials. The discovery of cutans in various plants has introduced another possible source of n(18) Hedburg, H. D. Am. Assoc. Pet. Geol. Bull. 1968, 52, 736-750.
alkanes upon thermal breakdown of this material.19 n-Alkane distributions in many crude oils typically maximize in the C20-C40 range (Figure 5a) but with the advent of high-temperature GC columns, the analysis of oils, bitumens, and waxes from a variety of source materials has shown the existence of compounds extending to at least C70 and beyond (Figure 5b). In summary, although oils from terrigenous sources are waxy, not all waxy oils are necessarily derived from terrigenous source materials.4-6 Isoprenoids. The ratio of the isoprenoids, pristane and phytane, has long been associated with the nature of depositional environments following the pioneering work of the Australian geochemists Brooks and Smith20 and Powell and McKirdy.21 Their concepts were reviewed and modified by Didyk et al.22 and then Haven et al.,23 who showed that in certain environments this ratio may also be affected by alternative sources for both pristane and phytane. In the early studies it was assumed that both pristane and phytane were derived from the phytol side chain of chlorophyll. More recently it has been shown that pristane can be derived from tocopherol and phytane from the bis(phytanyl ethers) (19) Telegaar, E. Resistant biomacromolecules in morphologically characterized constituents of kerogen: A key to the relationship between biomass and fossil fuels. Ph.D. Thesis, Technical University of Delft, Delft, The Netherlands, 1990, p 191. (20) Brooks, J. D.; Smith, J. W. Geochim. Cosmochim. Acta 1969, 3, 1183-1194. (21) Powell, T. G.; McKirdy, D. M. Nature 1973, 243, 37-39. (22) Didyk, B. M.; Simoneit, B. R. T.; Brassell, S. C.; Eglinton, G. Nature 1978, 271, 216-222. (23) ten Haven, H. L.; de Leeuw, J. W.; Rullko¨tter, J.; Sinninghe Damste´, J. S. Nature 1987, 330, 641-643.
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Figure 4. In order to obtain a more precise idea of the wax content of an oil, various concentration methods have been developed in order to quantitatively precipitate the wax fraction from the oil. This wax fraction can then be analyzed by high-temperature gas chromatography.
which occur in archeabacteria. Li et al.24 recently suggested that under certain conditions pristane may result from the catagenic decomposition of methyltrimethyltridecylchromans formed during diagenesis from condensation reactions between chlorophyll and alkylphenols. The use of the recently developed technique of combined gas chromatography-isotope ratio mass spectrometry (GCIRMS) has been used to determine whether or not two compounds such as pristane and phytane are derived from the same material. For example Freeman et al.25 have shown in the case of the Messel oil shale that these isoprenoids have different sources based on their isotopic values whereas for the Turrum oil from the Gippsland Basin, Australia, the isotopic values for the pristane (26.6‰) and phytane (26.9‰) were virtually identical. Many oils often contain high concentrations of long chain isoprenoids ranging up to at least C40 or C45 as determined by GCMS and single-ion monitoring of the ion at m/z 183. The majority of these compounds are regular isoprenoids or head-to-head isoprenoids commonly associated with archeabacteria. While the presence of these compounds may reflect the enhanced levels of microbial activity in the original oxic depositional environments, the possibility that the regular isoprenoids are also derived from polyprenols known to be associated with higher plant waxes cannot be excluded. Sesquiterpanes. Abundant concentrations of sesquiterpanes were unequivocally identified for the first (24) Li, M.; Larter, S. R.; Taylor, P.; Jones, D. M.; Bowler, B.; Bjoroy, M. Org. Geochem. 1995, 23, 159-167. (25) Freeman, K. H.; Hayes, J. M.; Trendel, J.-M.; Albrecht, P. Nature 1989, 353, 254-256.
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Figure 5. Ozocerite is a waxy fossil bitumen, and in this figure a comparison is made between the analysis of this material on both conventional and high-temperature gas chromatography. As noted above, the major fraction of this material is in a carbon number range that is not observed by conventional gas chromatography.
time in a number of Australian crude oils known to be sourced from terrestrial source material. Early work of Alexander et al.26,27 identified the major sesquiterpanes previously observed by Philp et al.28 as having drimane- and eudesmane-based structures, with more recent studies showing that these compounds extend to at least C24.29 In view of the apparent absence of eudesmanes in samples older than the Devonian Period, it was concluded initially that the eudesmanes were associated with an input from higher plants. The abundance of drimanes in the older sediments led to the conclusion that they were associated with a microbial input, although more recently drimane precursors have also been found in higher plants. The possibility of using these compounds as maturity indicators has not been investigated in detail but is something that may be potentially useful.30 Other sesquiterpanes such as the resin-derived cadalanes31 and cedrene and cuparene derivatives in Chinese oils known to contain a predominance of higher plant source material32 have also been identified. Diterpanes. Tri- and tetracyclic terpanes in the C19 and C20 range have long been known to be associated (26) Alexander, R.; Kagi, R.; Noble, R. J. Chem. Soc. Chem. Commun. 1983, 226-228. (27) Alexander, R.; Kagi, R.; Noble, R.; Volkman, J. K. Org. Geochem. 1984, 6, 63-70. (28) Philp, R. P.; Gilbert, T. D.; Friedrich, J. Geochim. Cosmochim. Acta 1981, 45, 1173-1180. (29) Wang, T.-G.; Simoneit, B. R.; Philp, R. P.; Yu, C. P. J. Energy Fuels 1990, 4 (2), 177-183. (30) Noble, R. A.; Alexander, R.; Kagi, R. J. Org. Geochem. 1987, 11, 151-156. (31) van Aarssen, B. G. K.; Hessels, J. K. C.; Abbink, O. A.; de Leeuw, J. W. Geochim. Cosmochim. Acta 1992, 56, 1231-1246. (32) Yongsong, H.; Ansong, G.; Fu, J.; Sheng, G.; Zhao, B.; Cheng, Y.; Li, J. Submitted to Advances in Organic Geochemistry 1991.
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beyerane, kaurane, and phyllocladane structures typically occur in gymnosperms. Kauranoid-type diterpenoids are probably a ubiquitous component of all angiosperms. In contemporary resins, primarane-type diterpenoids occur in conifers of the Pinaceae and Cupressaceae families and the southern conifer families of Podocarpaceae and Araucariaceae. Precursors of phyllocladane occur widely in the Podocarpaceae family and those of kauranes in the Podocarpaceae, Araucariaceae, and Taxodiaceae families. Diterpanes have also found limited use as maturity and depositional environmental indicators. Sesterterpanes. Significant concentrations of the 17,21-C24-secohopane were found in oils derived from terrigenous source materials, particularly Gippsland Basin oils from Australia,34 although this may be related to a relatively oxic depositional environment rather than a direct source indicator. More recent work has shown the presence of a variety of degraded compounds in the oils derived from terrigenous source materials, including those from New Zealand, Nigeria, Indonesia, and Taiwan36 and including the C24 des-A-ring analogues of the oleananes, lupanes, and ursanes which are typically very abundant in many of these oils (Figure 6). While it may be proposed that these des-A-ring compounds form via some method of A-ring degradation, it should be noted that a number of ring-A fissioned derivatives of pentacyclic triterpen-3-ols have been reported.37
Figure 6. Crude oils of Tertiary age derived from predominantly terrestrial source materials contain relatively high concentrations of des-A-sesterterpanes, which are thought to be derived from the C30 triterpanes associated with the angiosperm input to the source materials. In this figure, the des-A-sesterterpanes are the peaks labeled 1-4. The C30 triterpanes associated with the angiosperms are peaks 9, 11, 12, 14, and 16.
with resins from higher plants.33 The presence of their hydrocarbon analogues in oils and source rocks has been used to establish a higher plant input to the source rocks. Diterpanes are abundant in many oils known to contain higher plant materials from countries such as Australia, New Zealand, Taiwan, Papua New Guinea, and Indonesia in the southern Hemisphere.34-36 Diterpenoids based on the labdane, abietane, primarane, (33) Thomas, B. R. In Organic Geochemistry - Methods and Results; Eglinton, G., Murphy, M. T. J., Eds.; Springer-Verlag: Berlin, 1969; 599-618. (34) Philp, R. P.; Gilbert, T. D. In Advances in Organic Geochemistry 1985; Leythauser, D., Rullko¨tter, J., Eds.; Pergamon Press: London, England, 1986. (35) Alexander, R.; Larcher, A. V.; Kagi, R. I.; Price, P. L. APEA J. 1988, 310. (36) Woolhouse, A. D.; Oung, J.-N.; Philp, R. P.; Weston, R. J. Org. Geochem. 1992, 18, 23-31.
Pentacyclic Terpanes. Hopanes. The distribution of hopanes in oils is typically very simple although there are a large number of variations which can be related to depositional environment since virtually all the hopanes are derived from a bacterial source. In most oils and source rock extracts, the distributions are dominated by regular hopanes, maximizing at C30 with the concentration of the extended hopanes above C31 decreasing exponentially, characteristic of the distribution associated with more oxic-type environments. Methylhopanes and the various series of norhopanes identified in sample sets from other environments and source materials are generally absent or are in very low concentrations in oils derived from terrigenous sources although ten Haven et al.38 did report the occurrence of methylhopanes in coal samples. Oils derived from carbonate source rocks have very characteristic distributions with the C29 hopane, often being present in greater abundance than the C30 component since the C29 compound can be derived from both the regular hopane series and the 30-norhopane series (Figure 7). Hence if both series of hopanes are present, as they often are, in carbonate-derived oils, then the relative contribution of the C29 component will be significantly higher. In addition, variations in the distribution of the extended hopanes, beyond C31, often show a significant increase in the relative concentrations of the C35 components in oils from carbonate environments as a result of the highly reducing conditions during deposition. An index based on the relative distributions of the extended (37) Baas, W. J.; van Berkel, I. E. M.; Versluis, C.; Heerma, W.; Kreyenbro, M. N. Phytochemistry 1992, 31 (6), 2073-2078. (38) ten Haven, H. L.; Peakman, T. M.; Rullko¨llter, J. Geochim. Cosmochim. Acta 1992.
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Figure 7. m/z 191 chromatogram for an oil derived from a carbonate source rock. Characteristic terpane features of this type of oil include the following: relatively high concentrations of the C23 tricyclic terpane (1) and the C24 17,21-secohopane (2); the C29 hopane (5) . than the C30 (6) hopane; and the C35 extended hopane (11) > than the C34 extended hopane (10). Ts (3) and Tm (4) are the 18R(H)- and 17R(H)-C27-tris(norhopanes), respectively; C28 is the 29,30-bis(norhopane), and the peaks labeled 7-9 are the extended hopanes from C31 to C33.
hopanes was developed by Moldowan et al.39 and has been used extensively for correlation purposes. Non-hopanoid Terpanes. Non-hopanoid terpanes derived from higher plant sources are far more abundant than the hopanoids in many oils from terrigenous source materials. Higher plants contain a wide variety of triterpenoids and many of their hydrocarbon derivatives can be found in the corresponding oils and source rocks.40 These compounds are based on pentacyclic structures such as oleananes, lupanes, and ursanes, along with their demethylated analogues (Figure 6). These compounds are abundant in the oils of New Zealand,41 Taiwan,42 Beaufort-Mackenzie Basin, Canada,43 and Nigeria.36 Oleananes may be derived from a number of naturally occurring precursors, including taraxer-14-en-3β-ol and olean-12-en-3β-ol, which through a series of complex reduction and isomerization reactions can produce the 18R(H)- and 18β(H)-isomers of oleanane.45 Cadalane-type triterpanes are derived from precursors present in dammar resins and particularly abundant in oils from Indonesia.31,45 Although the predominant members of the series are in the C30 range, it has been shown that analogues of these compounds are present in the higher molecular weight region with components up to C60 and C75 and possibly higher. The presence of the saturated hydrocarbons is often accompanied by 1,6-dimethylnaphthalene and cadalene in the aromatic fraction. Steranes. Steranes in oils and source rock extracts are derived from naturally occurring sterols. Conver(39) Moldowan, J. M.; Lee, C. Y.; Sundararaman, T.; Salvatori, T.; Alajbeg, A.; Gjukic, B.; Demaison, G. J.; Slougui, N.; Watt, D. S. In Biological Markers in Sediments and Petroleum; Moldowan, J. M., Albrecht, P. A., Philp, R. P., Eds.; Prentice Hall: Englewood Cliffs, NJ, pp 370-396. (40) Whitehead, E. V. In Advances in Organic Geochemistry 1973; Tissot, B., Bienner, F., Eds.; Editions Technip: Paris, 1974; pp 225243. (41) Czochanska, Z.; Gilbert, T. D.; Philp, R. P.; Sheppard, C. M.; Weston, R. J.; Wood, T. A.; Woolhouse, A. D. Org. Geochem. 1988, 12, 123-135. (42) Philp, R. P.; Oung, J.-N. In Biological Markers in Sediments and Petroleum; (Moldowan, J. M., Albrecht, P., Philp, R. P., Eds.; Prentice Hall: Englewood Cliffs, NJ, 1989; pp 106-123. (43) Curiale, J. Chem. Geol. 1992, 93 (1/2), 21-46. (44) ten Haven, H. L.; Rullko¨tter, J. Geochim. Cosmochim. Acta 1988, 2, 2543-2548. (45) Grantham, P. J.; Posthuma, J.; Baak, A. In Advances in Organic Geochemistry 1981, Bjoroy, M., et al., Eds.; J. Wiley and Sons: New York, 1983; pp 675-683.
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Figure 8. m/z 217 chromatogram for a carbonate-derived oil showing a number of characteristic features for this type of oil. The peaks labeled 27, 28, and 29 are the regular series of steranes, and for the C29 homology, the 14β(H), 17β(H) isomers (2 and 3) and the 14R(H), 17R(H)-20(S) and -20(R) (1 and 4) epimers and isomers have been labeled.
sion pathways are complex and affected by both diagenetic and thermal reactions. However, in general, the carbon number distributions reflect the distributions of the sterols in the original source materials. Sterane distributions in oils derived predominantly from terrigenous source materials are very simple and are dominated by the C29 steranes which are in turn derived from the C29 sterols present in higher plants. However, in the situation with a mixed input of higher plant material and algal material, the problem is more complex since in addition to the C27 and C28 steranes, derived from algae, some of the C29 steranes may also be derived from the algal material46 (Figure 8). Hence in many cases the sterane distributions are more commonly used for correlation purposes rather than trying to provide an absolute determination of the nature of the source material. Another characteristic feature in oils and source rock extracts is the presence of diasteranes. It was proposed initially that the concentration of diasteranes relative to the steranes reflected the presence of clay minerals and their ability to catalyze sterene rearrangement reactions. More recently it has become apparent that the oxicity of the depositional environment provides an additional clue as to the fate of the sterols since in a highly anoxic, or reducing, environment the sterenes formed from the sterols will be reduced, thus reducing the amount of sterenes available for the rearrangement reaction. In a more oxic-type environment less of the sterenes will be reduced and available for rearrangement to diasterenes. In summary, it can be proposed that the concentration of the rearranged steranes in these oils more closely reflects the oxicity of the environment rather than the presence or absence of the clay minerals.47 Methylsteranes and Dinosteranes. 24-n-Propylcholestanes (4-desmethyl), derived from the corresponding sterols, are unique to the Chrysophyte family of marine algae, and it has been proposed that the ratio of these C30 steranes/total steranes varies along with the ratio of the C28 steranes/total steranes and can be used for characterization of marine source materials.48 The n-propyldesmethylsteranes first appeared in the Early (46) Volkman, J. K. Org. Geochem. 1986, 9, 83-99. (47) Moldowan, J. M.; Sundararaman, P.; Schoell, M. Org. Geochem. 1985, 10, 915-926. (48) Moldowan, J. M.; Fago, F. J.; Lee, C. Y.; Jacobson, S. R.; Watt, D. S.; Slougui, N. E.; Jeganathan, A.; Young, D. C. Science 1990, 247, 309-312.
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Paleozoic, and the precursor sterols are thought to have appeared in the precursor algae in the early Ordovician and Devonian periods.48 McCaffrey et al.49 recently discovered the presence of 24-isopropylcholestanes in oils and bitumens from Early Proterozoic (approximately 1800 mybp) to Miocene (approximately 15 mybp) Age marine strata. The relative abundance of the 24isopropylcholestanes to the 24-n-propylcholestanes varies with source rock age and provides a means for constraining the age of a source rock from which a specific oil may have been derived. Late Proterozoic (Vendian) and Early Cambrian oils and/or bitumens from Siberia, the Urals, Oman, Australia, and India have a high ratio of 24-isopropylcholestanes to 24-npropylcholestanes (>1) while younger and older samples have a lower ratio (
